Two-way optical transmission system

ABSTRACT

A two-way optical transmission system in which a plurality of light signals of wavelengths differing one from another are transmitted through a single optical fiber from one end thereof to the other end and from the other end to the one end. One or more assemblies each including an electrooptic transmitter, a dielectric thin-film filter and a rod lens and one or more assemblies each including a rod lens, a dielectric thin-film filter and an opto-electric receiver are connected to the single optical fiber at both ends thereof, respectively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bidirectional or two-way opticaltransmission system designed and suited for transmission light signalsof wavelengths differing from one another in up-link and down-linkdirections, respectively, by using a single optical fiber transmissionline.

2. Description of the Prior Art

The analogue transmission system in which a semiconductor laser diode(hereinafter referred to as laser diode or LD) is employed promises tobe applicable to the transmission of image information or data ofindustrial television, high-quality television, cable television and thelike systems. However, in the transmission system in which the laserdiode is used in combination with a multi-mode optical fiber, therearises a problem that so-called speckle noise and speckle distortionappear due to mutual interference among different transmission modes oflaser light because of the interferential nature thereof. For thisreason, the possible maximum transmission distance is limited to a rangeof 1 to 2 km at best in the present state of the art. In contrast, in atransmission system in which the laser diode is used in combination witha single-mode optical fiber, the speckle noise and speckle distortionmentioned above can be evaded, whereby a long distance transmissionaround 10 km is expected to be realized. It should however be pointedout that in the transmission system employing the single-mode opticalfiber, the two-way transmission has not yet been realized at present,which can be explained by the facts that the optical multiplexer of lowloss is difficult to be realized and that the transmission system has avery complicated structure and suffers from degradation in thetransmission quality, low reliability and is very expensive.

FIG. 1 of the accompanying drawings shows a general arrangement of atwo-way optical transmission system which is realized by using thosedevices and components which are commercially available at present.Referring to FIG. 1, it is assumed that the path of a light signal (ofwavelength λ₁) propagating through an optical fiber 7 in the directionindicated by an arrow 8 is referred to as an up-link, while the path ofa light signal (of wavelength λ₂) traveling in the direction indicatedby an arrow 8' is termed the down-link. In the up-link circuit, a signalapplied to information or data input terminal 15 is supplied to a laserdiode drive circuit 14 (hereinafter also referred to as LD drivecircuit). A laser diode or LD 1 is driven by the output signal of the LDdrive circuit 14. The oscillation wavelength of the LD 1 is representedby λ₁. The laser light emitted by the LD 1 passes through a sphericallens 2 and a graded index (GRIN) rod lens 3 (having a length of about1/4 pitch, where one pitch corresponds to a period of a meanderingoptical path within the rod lens) and enters an optical fiber 4. Havingtraveled in the direction indicated by an arrow 5, the laser lightreaches an optical multiplexer 6. The output light signal from theoptical multiplexer 6 travels through the optical fiber 7 in thedirection indicated by the arrow 8 to reach a photoelectric oroptoelectric receiver 11 after having passed through an opticalmultiplexer 6' and an optical fiber 9. The electric output signalproduced by the receiver 11 is, after having been amplified by anamplifier 12, demodulated by a demodulator 13, whereby the originalinformation or data signal is reproduced. In the case of the down-linkcircuit, a signal applied to a data input terminal 15' is applied to alaser diode drive circuit 14', the output signal of which drives a laserdiode or LD 1' which then oscillates at the wavelength λ₂. The laserlight emitted by the LD 1' travels through a spherical lens 2', a gradedindex (GRIN) type rod lens 3' (having a length of about 1/4 pitch), anoptical fiber 4', an optical multiplexer 6', the optical fiber 7 andthen the optical multiplexer 6 in this order and reaches a photoelectricreceiver 11' through an optical fiber 9', as indicated by an arrow 10'.A corresponding electric signal produced by the receiver 11' isamplified by an amplifier 12' and subsequently demodulated by ademodulator 13' to the original data signal. As will be seen, thetransmission system described above requires a great number of devicesor components. Among them, the optical multiplexers, the sphericallenses and the graded index type rod lenses have to be provided inpairs, respectively. Besides, adjustment for alignment of the opticalaxes of the individual components is obviously very troublesome,involving difficulties in implementing the optical transmission systemwith low loss and high reliability at reasonable costs. Furthermore,reflected light rays from the various devices or elements will possiblybe re-injected into the LD, bringing about changes in the longitudinalmode which in turn will produce crosstalk noise. To evade such unwantedphenomenon, corresponding measures must be taken against the lightreflections by the various devices, which means that the systemstructure will then be considerably complicated.

FIG. 2 shows a typical structure of the hitherto known opticalmultiplexer corresponding to the one denoted by the numerals 6 and 6' inFIG. 1. Referring to FIG. 2, the light of wavelength λ₁ travelingthrough the optical fiber 4 in the direction indicated by an arrow 5 isapplied to a rod lens 17 and hence to a pentagonal prism 21 in which thelight is reflected by a dielectric mirror 19 to a dielectric thin-filmfilter 20 at which the light is again reflected toward a rod lens 18.The light signal leaving the rod lens 18 travels through the opticalfiber 7 in the direction indicated by an arrow 8. On the other hand,light of wavelength λ₂ propagates through the optical fiber 7 in thedirection indicated by an arrow 8' and passes through a dielectricthin-film filter 20 to be transmitted through a rod lens 16 to theoptical fiber 9' in which the light signal travels in the directionindicated by an arrow 10'. In this way, the light signal of wavelengthλ₁ and the light signal of wavelength λ₂ are separated or demultiplexedfrom each other. However, the optical multiplexer shown in FIG. 2requires a great number of optical elements and a complex structure,presenting an obstacle to the realization at low costs and with highreliability. Further, because of the single-mode transmission, the corediameter of the optical fiber has to be smaller than 10 μm, wherebyextremely strict accuracy requirements are imposed with regard to thepositioning of the optical fibers. As the loss of the opticalmultiplexer itself as well as loss involved in the couplings between theLD and the lens, between the lens and the optical fiber and between theoptical fiber and the optical multiplexer is increased, the transmissiondistance is correspondingly shortened, making it difficult to realize along distance transmission. For the reasons described above, the two-wayoptical transmission system in which a single-mode optical fiber is usedis not yet realized for practical applications. By the way, in theone-way transmission system, the transmission distance is only around 5km.

SUMMARY OF THE INVENTION

An object of the present invention is to realize a two-way opticaltransmission system in an extremely simplified structure in which asingle-mode optical fiber can be employed, to thereby make possible along distance data transmission.

In view of the above object, there is provided according to a generalaspect of the present invention a two-way optical transmission system inwhich data transmission is carried out by using light signals ofdifferent wavelengths in the up-link direction and the down-linkdirection, respectively, through a first single optical fiber, whereinlight of a wavelength λ₁ transmitted from a light signal transmitter ofthe up-link circuit is introduced to the first single optical fiber atone end thereof through a dielectric thin-film filter which passestherethrough only the light of wavelength λ₁ and a rod lens, while lightof wavelength (λ₂) transmitted from a light signal transmitter of thedown-link circuit is introduced to the first single optical fiber at theother end through a dielectric thin-film filter which passes only thelight of the wavelength λ₂ and a rod lens. The light beams of thedifferent wavelengths λ₁ and λ₂ leaving the aforementioned singleoptical fiber are reflected, respectively, by the two dielectricthin-film filters mentioned above to enter second optical fibersdisposed, respectively, adjacent to the end faces of the rod lenseswhich are located, respectively, at the ends of the first optical fiber.The information signal is introduced to the light signal receiverthrough the associated second optical fiber to be demodulated.

According to the invention, the rod lens which serves to introduce lightemitted by a light emission element into the single optical fiber isprovided with the dielectric thin-film filter for effecting the two-waytransmission, as the result of which the heretofore required opticalmultiplexer of the complicated structure can be spared. In other words,the present invention proposes a novel two-way optical transmissionsystem in which both the function of coupling light emitted by the lightemission element to the optical fiber and the function of the opticalmultiplexer/demultiplexer are realized by the single rod lens.Accordingly, the two-way optical transmission system according to theinvention is of an extremely simplified structure and can enjoy asignificant loss reduction and hence a long distance transmission. Bythe way, with the phrase "oscillation wavelength (λ₁, λ₂)" it isintended to mean the center wavelength of oscillation. The length of therod lens is about n/4 pitch (where n=1, 3, 5 . . . ).

According to a preferred embodiment of the invention, a two-waytransmission in which three or more different light wavelengths are madeuse of can be realized by providing the three or more opticaltransmitter units and optical receiver units, respectively, which aremutually interconnected by the optical fibers in the manner describedabove.

In this conjunction, a plurality of the dielectric thin-film filterswhich exhibit different optical characteristics (transmission andreflection characteristics), respectivly, may be provided in n layerswith angles θ_(n) which differ for the n layers, respectively, at an endface of the rod lens which is located on the entrance side of the lightemitter by the light emission element or LD. On the other hand, theopposite end face of the rod lens is coupled with one end of n opticalfibers in addition to the single optical fiber serving for the up- anddown-transmission of the signals. The other ends of n optical fibers arecoupled to optical transmitter or receiver units for transmitting orreceiving light signals of other oscillation wavelengths. In this way,there can be realized the two-way transmission of three or more lightsignals of the wavelengths which differ from one another.

The present invention can be applied not only to the single-mode opticalfiber transmission system but also to a multi-mode optical fibertransmission system.

It is also another object of the present invention to simplify theadjustment required for the coupling between the optical fiber and theoptical transmitter unit or optical receiver unit, to thereby reduce thetime taken for the adjustment and realize a high degree of coupling,inexpensiveness and reduction in loss.

In view of the object mentioned above, it is proposed according toanother aspect of the invention that an optical fiber holding means isprovided on the light exit side of the rod lens, wherein the optical up-and down-transmission fiber having the end disposed at the rod lens andone or more optical fibers juxtaposed around the former for transmittinglight signals demultiplexed or multiplexed by the dielectric thin-filmfilter are fixedly secured integrally on the light exit side of theoptical fiber holding means. According to a further feature of thepresent invention, the rod lens provided with one or more dielectricthin-film filters may be combined with the optical fiber holding meansto thereby constitute an optical module which is disposed at the optical(electrooptic) transmitters of the up-link circuit and the down-linkcircuit, respectively. The optical fiber holding means is so constitutedthat it accommodates therein the optical fiber for transmitting thelight signal passing through the dielectric thin-film filter and theoptical fiber for transmitting the light signal demultiplexed ormultiplexed through the dielectric thin-film filter, wherein each fiberis coupled with the transmission path and the transmitter or thereceiver to thereby effect the two-way transmission of two or more lightsignals of different wavelengths.

It is a further object of the present invention to provide a structureof the optical module for the two-way transmission which can be packagedwith an improved accuracy without consuming time. With the invention, itis also contemplated to provide a method of manufacturing the opticalmodule.

In view of the above object, it is proposed according to another aspectof the invention that one end face of the rod lens is ground slant at adesired angle, and that upon formation of the dielectric thin-filmfilter on the ground end face, a mark indicating that the grinding hasbeen effected along the Y--Y' axis of that end face is left on the endface of the rod lens or alternatively the rod lens is previouslyinserted and secured within a cylindrical tube which has a mark on theY--Y' axis and subsequently the grinding and the formation of thedielectric thin-film are performed with reference to the mark. Areceptacle base is prepared which allows the packaging and assembling tobe carried out with reference to the mark, to thereby realize theoptical module for the two-way optical transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a general arrangement of a two-wayoptical transmission system which is composed of hitherto known devicesor components;

FIG. 2 is a schematic view showing a structure of a hitherto knownoptical multiplexer;

FIG. 3 is a view showing schematicallly a general arrangement of atwo-way optical transmission system according to an exemplary embodimentof the present invention;

FIG. 4 is a view similar to FIG. 3 and shows a two-way opticaltransmission system according to another embodiment of the invention;

FIG. 5 and FIGS. 6A and 6B are views showing various structure ofdevices which exhibit optical coupling function and opticalmultiplexing/demultiplexing function and which are employed forembodying the present invention;

FIG. 7 is a view showing a general arrangement of a two-way opticaltransmission system according to another embodiment of the invention;

FIGS. 8A to 8D are views for graphically illustrating wavelengthvis-a-vis optical attenuation characteristics of dielectric thin-filmfilters employed in the system shown in FIG. 7;

FIG. 9 is a view similar to FIG. 7 and shows a two-way opticaltransmission system according to a further embodiment of the invention;

FIGS. 10A to 10D and 11A to 11C are views showing various structures ofthe devices which are imparted with the optical coupling function andthe optical multiplexing/demultiplexing function according to theteachings of the invention;

FIG. 12 is a block diagram showing a general arrangement of a two-wayoptical transmission system according to another embodiment of theinvention;

FIGS. 13A and 13B are views for illustrating a structure of an opticalfiber holder used in the system shown in FIG. 12;

FIGS. 14A and 14B are views corresponding to FIGS. 13A and 13B and showthe optical fiber holder in the state in which optical fibers areinserted in the holder;

FIGS. 15A, 15B, 16A and 16B show, respectively, other possiblestructures of the optical fiber holder according to further embodimentsof the invention;

FIG. 17 is a vertical sectional view showing a structure of an integraloptical module according to another embodiment of the invention;

FIGS. 18A to 18D are views for illustrating stepwise formation ofdielectric thin-film filters on ends of a rod lens inserted and securedwithin a cylindrical tube having a mark and subsequently ground slantaccording to the teaching of the invention;

FIGS. 19A and 19B are views showing an optical module for a two-wayoptical transmission according to a further embodiment of the invention;

FIGS. 20A and 20B are views showing, respectively, a rod lens insertedin a cylindrical tube and a receiving element according to a stillfurther embodiment of the invention;

FIGS. 21A and 21B are views showing versions of the structures shown inFIGS. 20A and 20B;

FIGS. 22A and 22B are views showing the optical fiber holder accordingto still another embodiment of the present invention;

FIG. 23 is a view showing an optical module for the two-way opticaltransmission according to yet another embodiment of the invention;

FIGS. 24A and 24B are views showing a structure of the rod lens attachedwith a mark; and

FIG. 25 is a block diagram showing an arrangement of a two-way opticaltransmission system according to a further embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows a general arrangement of a two-way optical transmissionsystem according to an exemplary embodiment of the present invention.This system differs from the one shown in FIG. 1 in that each rod lens22 or 24 having a length of about 1/4 pitch is ground slant or bevelled(i.e. with inclination) at one or both ends, wherein a dielectricthin-film filter 23 or 25 imparted with desired optical charcteristicsis formed on the one ground end face, and two optical fibers aredisposed closely adjacent to each other at the other end face of the rodlens, and that the optical multiplexers 6 and 6' are omitted in thetwo-way transmission system shown in FIG. 3. The filter 23 isconstituted by a multi-layer dielectric film which passes light of awavelength λ₁ and reflects light of another wavelength λ₂. On the otherhand, the filter 25 is constituted by a multi-layer dielectric filmwhich reflects the light signal of the wavelength λ₁ while transmittingthe light signal of the wavelength λ₂. The filter may be realized in aband pass filter or a short wavelength band (or long wavelength band)pass filter.

Next, operation of the two-way optical transmission system shown in FIG.3 will be described. The laser light (of wavelength λ₁) emitted by alight emission element or laser diode (LD) 1 is collimated by aspherical lens 2 and enters the dielectric thin-film filter 23. In thecase of the illustrated system, the light emission element 1 and thespherical lens 2 constitute an optical (i.e. electrooptical)transmitter. Light signal incident on the dielectric thin-film filter 23passes therethrough and travels through the rod lens 22 to be focusedinto an optical fiber 7 for transmission therethrough in the directionindicated by an arrow 8 and enters the rod lens 24. The light ofwavelength λ₁ incident upon the rod lens 24 reaches the dielectricthin-film filter 25 to be reflected thereby. The reflected light isreversely directed and focused onto the entrance end face 27 of the rodlens 24 at a position which is deviated from the light entrance orincidence position by a distance corresponding to an angle θ₂ at whichthe end face of the rod lens covered with the dielectric thin-filmfilter 25 is slanted. Thus, by positioning the inlet port of an opticalfiber 9 at the position where the reflected light of wavelength λ₁ isfocused, the reflected light signal travels through the core of theoptical fiber 9 in the direction indicated by an arrow 10 to be receivedby a photoelectric or optoelectric transducer 11 whose electrical outputsignal is amplified by an amplifier 12 and finally demodulated to anoriginal information or data signal by a demodulator 12'. It should bementioned that in the case of the illustrated system, the photoelectricor optoelectric transducer 11, the amplifier 12 and the demodulator 13constitute an optoelectric or optical receiver. On the other hand, thelaser light of the wavelength λ₂ is collimated by means of a sphericallens 2' and enters the dielectric thin-film filter 25 which passes theincident light. The light signal having passed the filter 25 travelsthrough the rod lens 24 and is focused into a core of the optical fiber7 disposed closely adjacent to the end face 27 of the rod lens 24 totravel through the optical fiber 7 in the direction indicated by anarrow 8' and enter the rod lens 22. The light signal incident on the rodlens 22 reaches the dielectric thin-film filter 23 to be reflectedthereby toward the end face 26 of the rod lens 22. The reflected lightis focused onto the end face 26 at a position which is deviated from theposition of the incident light by a distance corresponding to an angleθ₁ at which the end face of the rod lens 22 covered with the filter film23 is bevelled or inclined. By positioning the core of an optical fiber9' at a point where the reflected light is focused, the reflected lightof wavelength λ₂ leaving the rod lens 26 is transmitted through theoptical fiber 9' in the direction indicated by an arrow 10' to bereceived by a photoelectric or optoelectric transducer 11' whose outputsignal is demodulated to the original data signal by a demodulator 13'after having been amplified by an amplifier 12'.

As will be seen from the above description, the two-way opticaltransmission system according to the invention in which light signals ofdifferent wavelengths are used can be realized in an extremelysimplified structure as compared with the arrangement of the two-waytransmission system shown in FIG. 1. This transmission system isadvantageous in that the number of the requisite component devices issignificantly decreased, low-loss transmission can be attained,troublesome adjustment for the optical axis alignment is reduced infrequency, and that the present system promises realization of a longdistance transmission. Additionally, by virtue of such feature that therod lens (22, 24) is cut slant at one or both ends thereof, the lightreflected at the slanted end face of the rod lens as well as lightreflected at the end face of the optical fiber is positively preventedfrom being re-injected to the light emission element or LD to anadvantage, as the result of which the crosstalk noise can be effectivelysuppressed to another advantage. The angles θ₁ and θ₂ may be selected ina range of a fraction to thirties in degree. It will be understood thatas the angle is selected larger, the distance between the juxtaposedoptical fibers 7 and 9' (or 9) can be increased. In other words, thedistance between the juxtaposed optical fibers is in proportion to theangle mentioned above. By selecting the angle θ₁ equal to θ₂, the rodlens is implemented in a configuration symmetrical to the centralvertical axis as viewed in the drawing. Further, in case the dielectricthin-film filter (23, 25) is implemented in a band pass filter,transmission of unwanted light signals through the optical fiber can beeffectively suppressed by narrowing extremely the bandwidth of thefilter. (To this end, a plural stages of filters may be connected incascade or filters may also be provided on the reflected lighttransmitting end faces 26 and 27 of the rod lenses 22 and 24.) The abovedescription has been made on the assumption that the single-mode opticalfiber is employed. However, the invention can be equally applied to thesystem in which a multi-mode optical fiber is adopted. Further, a lightemission diode (LED) may be employed as the light emission element inplace of the laser diode (LD).

In another version of the transmission system shown in FIG. 3, it ispossible to use a single-mode optical fiber only for the opticaltransmission fiber 7, while a multi-mode optical fiber of a large corediameter can be used as the optical fibers 9 and 9', provided that thelength of the latter is within several meters. In that case, the amountof light input to the photoelectric transducer of the receiver can beincreased. In another modification, the optical fibers 9 and 9' may beremoved by disposing the photoelectric transducers 11 and 11' in frontof the associated end faces 26 and 27 of the rod lenses 22 and 24 at thefocal points of the reflected light, respectively, with or withoutinterposition of lenses. In that case, an integral transmitter/receivermodule for the two-way optical transmission can be realized in which thelaser diode (LD) and the photoelectric transducer are integrallyincorporated. In should be added that the distance between the end faceof the laser diode 1 and the spherical lens 2 is selected equal to thefocal length of the spherical lens 2 and the distance between thespherical lens 2 and the rod lens 22 is selected equal to the sum of thefocal lengths of these lenses. Same applies true to the distance betweenthe LD 1' and the spherical lens 2' and the distance between thespherical lens 2' and the rod lens 24. The focal lengths of thespherical lenses and the rod lenses are determined in dependence on thespot size of the optical fiber and the diameter of light beam emitted bythe laser diode.

FIG. 4 shows a two-way optical transmission system according to anotherembodiment of the invention which is designed for the two-waytransmission of three light signals differing from one another in thewavelength. More specifically, the two-way optical transmission systemshown in FIG. 4 is so arranged that a light signal of wavelength λ₁originating in a laser diode or LD 1 and a light signal of wavelength λ₃originating in a LD 1" are transmitted through an optical fiber 7 in thedirection indicated by an arrow 8 (up-link direction), while a lightsignal of wavelength λ₂ emitter by a LD 1' is transmitted through theoptical fiber 7 in the direction indicated by an arrow 8' (downlinkdirection). In this arrangement, a dielectric thin-film filter 23' is sodesigned as to pass the light of wavelength λ₁ while reflecting thelight signals of the wavelengths λ₂ and λ₃. On the other hand, adielectric thin-film filter 25' is designed to pass only the light ofwavelength λ₂ and reflect the light signals of wavelengths λ₁ and λ₃. Adielectric thin-film filter 28 passes the light of wavelength λ₃ andreflects the light of wavelength λ₂. Further, a dielectric thin-filmfilter 31 is designed to pass the light of wavelength λ₃ and reflect thelight of wavelength λ₁.

Next, operation of the two-way optical transmission system shown in FIG.4 will be described.

The light signal of wavelength λ₁ emitted by the laser diode or LD 1 istransmitted through the optical fiber 7 in the direction indicated bythe arrow 8, as mentioned above, and reflected by the dielectricthin-film filter 25' deposited on the end face of the rod lens to enterthe optical fiber 9 to be transmitted therethrough in the directionindicated by the arrow 10. The light leaving the optical fiber 9 entersthe rod lens 30 and undergoes reflection at the dielectric thin-filmfilter 31 to be subsequently transmitted through the optical fiber 35 inthe direction indicated by an arrow 36 and received by the photoelectrictransducer 11 whose output signal is amplified by the amplifier 12 andthen demodulated to the data signal by the demodulator 13. On the otherhand, the light signal of wvelength λ₃ emitted by the LD 1" isintroduced to the optical fiber 9' in the focused state after havingpassed through the spherical lens 2", the dielectric thin-film filter 28and the rod lens 29, to be transmitted through the optical fiber 9' inthe direction indicated by an arrow 10"0 to enter the rod lens 22. Thelight signal reaches the dielectric thin-film filter 23' and isreflected to the optical fiber 7 to be transmitted to the dielectricthin-film filter 25' as in the case of the light of wavelength λ₁. Thelight signal (λ₃) is reflected by the filter 25' to be coupled to theoptical fiber 9 and transmitted to the photoelectric transducer 11"through the rod lens 30, the dielectric thin-film filter 31 and thespherical lens 32. The output signal of the photoelectric transducer 11"is demodulated to the data or information signal by the demodulator 13"after having been amplified by the amplifier 12". Finally, the lightsignal of wavelength λ₂ sent out from the LD 1' is transmitted throughthe optical fiber 7 in the direction indicated by a broken line arrow 8'to be reflected by the dielectric thin-film filter 23' to the opticalfiber 9' through which the light signal λ₂ is transmitted in thedirection indicated by a broken line arrow 10' to be again reflected bythe dielectric thin-film filter 28, whereby the light signal λ₂ istransmitted through the optical fiber 33 in the direction indicated by abroken line arrow 34 to reach the photoelectric transducer elemnent 11'.Through the photoelectric conversion of the light signal λ₂ by thetransducer 11', the corresponding electric signal is produced by thelatter, amplified by an amplifier 12' and demodulated by a demodulator13, whereby original data or information signal is obtained. In thismanner, the two-way transmission of three light signals differing in thewavelength can be effected in the system shown in FIG. 4. It is apparentthat the two-way transmission for the four or more light signals ofdifferent wavelengths can be realized in a similar manner.

In the case of the two-way transmission system shown in FIG. 7, theoptical fiber may be either a single-mode optical fiber or a multi-modeoptical fiber. In the case of the single-mode optical fibertransmission, the optical fibers 7, 9 and 9' may be constituted by thesingle-mode optical fiber, while the multi-mode optical fiber may beemployed as the optical fibers 33 and 35 to thereby increase theefficiency of the optical coupling to the photoelectric transducers 11and 11'. Further, instead of employing the optical fibers 33 and 35, thephotoelectric transducer elements 11 and 11' may be bonded to the exitend faces of the rod lenses 29 and 30, respectively, (with or withoutinterposition of a lens). By the way, with the phrase "photoelectrictransducer" used herein, it is intended to represent and encompassphotoelectric or optoelectric elements such as photodiode (PD),avalanche photodiode (APD) or the like which may be equipped with apreamplifier, if necessary.

FIG. 5 shows in an enlarged view a structure for introducing lightemitted by the semiconductor light emission element to the optical fiberand an arrangement capable of performing the optical demultiplexingfunction according to the teaching of the present invention. In thearrangement shown in FIG. 5, the rod lens 22 of the system shown in FIG.3 is divided into a pair of rod lenses 37 and 38, wherein a dielectricthin-film filter 23 is interposed between the rod lenses 37 and 38. Inthis arrangement, the length l₁ +l₃ is substantially equal to 1/4 pitch.The length l₂ should preferably be smaller than 1/10 pitch. The ends ofthe optical fibers 7 and 9' are intimately contacted to the end face ofthe rod lens 38. By adjusting the length l₂ and the angle of inclinationof the dielectric thin-film filter 23, the degree of the opticalcoupling to the optical fibers can be adjusted at optimum.

The foregoing description has been made on the assumption that thespherical lenses 2, 2', 2" and 32 are used. However, it should beappreciated that cylindrical lenses may be used in place of thesespherical lenses. In other words, any type of lens can be used so far asit can convert the light emitted by the light emission element to thecollimated light beam. In the case of the illustrated system describedabove, the dielectric thin-film (multi-layer thin-film) filter isdisposed on the slantingly ground end face of the rod lens. As a versionof such disposition, it is also possible that a glass spacer 39 isdisposed on one end face of the rod lens 22, wherein the dielectricthin-film filter 23 is formed on the exposed surface of the glassspacer, as is shown in FIG. 6A. Alternatively, a glass spacer 39deposited with the dielectric thin-film filter may be disposed in frontof the associated end face of the rod lens 22 with a small spacetherebetween, as is illustrated in FIG. 6B. Further, the end faces ofthe individual devices or elements (such as, for example, optical fiberand rod lens) may be applied with an anti-reflecting coating (ARcoating) which is conventionally employed. In that case, the free end ofthe optical fiber and the end face of the rod lens facing in oppositionto the optical fiber end (e.g. the end faces 26 and 27 in FIG. 4) may beground slantingly or bevelled at a desired angle (in a range of afraction to tens degrees) and the anti-reflecting coating may be appliedthereon. In that case, the near-end crosstalk can be significantlyreduced. Further, an isolator may be inserted for isolating the laserdiode and the optical fiber.

As the measure for suppressing the reflected wave tending to return tothe laser diode or LD from the system coupled thereto, the associatedlens may be vertically offset relative to the LD in addition to theslantingly grinding of the end faces of the individual elements. In thisconjunction, when the lens is disposed slightly inclined relative to theoptical axis, the reflected wave can be suppressed effectively. In thearrangement shown in FIG. 6B, the end face of the lens 22 may berounded.

In the case of the two-way optical transmission system in which themulti-mode optical fiber is adopted, the spherical lenses 2, 2', 2" and32 may be spared. More specifically, since the core diameter of themulti-mode optical fiber is several tens μm which is five or more timesas large as the core diameter of the single-mode optical fiber, it isreadily possible to focus the light emitted by the light emissionelement or LD into the multi-mode optical fiber by adjusting thedistance between the rod lens and the optical fiber. Of course, thefocusing can be attained with a much improved accuracy by using thespherical lens, which however is inexpensive from the economicalstandpoint.

Upon packaging the LD and the spherical lens, they are usuallyhermetically accommodated within a container provided with an opticalwindow through which the light power is outputted. In that case, thedielectric thin-film filter may be applied onto the inner surface of theoptical window through vapor deposition or bonding. Then, thecharacteristics of the dielectric thin-film filter can be effectivelyprotected from deterioration which may otherwise take place due tovariations in temperature and humidity.

As will now be seen, by providing the dielectric thin-film filter incombination with the rod lens which serves to introduce the lightemitted by the semiconductor light emission element into the opticalfiber according to the teaching of the invention, the optical couplingfunction as well as the optical branching or demultiplexing function canbe realized, as the result of which the heretofore required provision ofthe optical demultiplexer of a complicated structure requiring a largenumber of components is rendered unnecessary, whereby the two-wayoptical system of an extremely simple structure and low loss can beobtained.

The present invention can be applied not only to the analogue signaltransmission but also to the digital signal transmission.

FIG. 7 shows a general arrangement of the two-way optical transmissionaccording to another embodiment of the invention, in which three lightsignals of different wavelengths are transmitted. The system shown inFIG. 7 differs from the one shown in FIG. 4 in that the end faces of therod lenses 22 and 24 of about 1/4 pitch in length on which the lightemitted by the associated LD impinges are each provided with a pluralityof dielectric thin-film filters 23a, 23b; 25a, 25b imparted with desiredoptical characteristics, wherein end portions of three optical fibersare disposed in juxtaposition closely to the opposite or other end facesof the rod lenses, respectively. Referring to FIG. 7, two light signalshaving the wavelengths λ₁ and λ₃, respectively, are transmitted throughthe optical fiber 7 in the up-link direction indicated by an arrow 8,while the other light signal of wavelength λ₂ is transmitted through theoptical fiber 7 in the down-link direction indicated by an arrow 8'. Inother words, the system shown in FIG. 7 is a three-wave two-waytransmission system in which two light signals are transmitted in theup-link directon with the other light signal being transmitted in thedown-link direction. In conjunction with the illustrated system, FIGS.8A to 8D graphically show optical characteristics of the individualdielectric thin-film filters 23a, 23b, 25a and 25b, respectively. Inthese figures, wavelength is taken along the abscissa, while attenuationof light is taken along the ordinate. More specifically, FIG. 8Aillstrates the optical characteristics of the dielectric thin-filmfilter 23a. It will be seen that this filter 23a passes the light signalof wavelength λ₁ and reflects the light signals of wavelengths λ₂ andλ₃. FIG. 8B shows the optical characteristics of the dielectricthin-film filter 23b which passes the light signals of wavelengths λ₁and λ₃ and reflects the light signal of wavelength λ₂. FIG. 8C shows theoptical characteristics of the dielectric thin-film filter 25a whichpasses the light signals of wavelengths λ₁ and λ₂ and reflects the lightsignal of wavelength λ₃. Finally, FIG. 8D shows the opticalcharacteristics of the dielectric thin-film filter 25b. As will be seen,this filter 25b characteristically passes the light signal of wavelengthλ₂ and reflects the light signals of wavelengths λ₁ and λ₃. Thedielectric thin-film filters imparted with the optical characteristicsmentioned above can be realized by resorting to hitherto knowntechniques. Referring to FIG. 7, the laser light (of wavelength λ₁)emitted by the laser diode or LD 1 is collimated by the spherical lens 2and transmitted through the dielectric thin-film filter 23a, a glassspacer 60, the dielectric thin-film filter 23b and the rod lens 22 to befocused into the optical fiber 7 and travels therealong in the directionindicated by the arrow 8 to enter the rod lens 24. The light signal ofwavelength λ₁ traveling through the rod lens 24 and impinging on thedielectric thin-film filter 25a passes through it and the glass spacer62 to impinge on the dielectric thin-film filter 25b. The light signalof wavelength λ₁ is then reflected by the filter 25b to propagate alongthe reverse path through the glass spacer 62, the dielectric thin-filmfiler 25a and the rod lens 24. Since the dielectric thin-film filter 25bis inclined at the angle θ₁, the position on the end face 27 of the rodlens 24 at which the reflected light signal (λ₁) is focused is deviatedfrom the position of which it enters the rod lens 24 by a distance whichis in proportion to the angle θ₁. This deviation d₁ can be approximatedby the following expression: ##EQU1## where n_(o) represents therefractive index of the rod lens 24 and √A represents the refractiveindex distribution (gradation) constant.

By disposing the optical fiber 9 at the position where the reflectedlight signal of wavelength λ₁ is focused, the reflected light signaltravels through the core of the optical fiber 9 in the directionindicated by an arrow 10 to be received by a photoelectric transducer 11whose electrical output signal is amplified by an amplifier 12 anddemodulated to the original data signal by a demodulator 13. With thephrase "photoelectric transducer", it is intended to mean an elementsuch as photodiode, avalanche photodiode and the like. The transducerelement may be equipped with a preamplifier. On the other hand, thelaser light of wavelength λ₃ emitted by the LD 1" is collimated by thespherical lens 2" before entering the rod lens 29. The light signalhaving traveled through the rod lens 29 is focused into the opticalfiber 9' to propagate therethrough in the direction by an arrow 10" andreach the rod lens 22. The light signal of wavelength λ₃ incident on therod lens 22 passes through the dielectric thin-film filter 23b and theglass spacer 60 to reach the dielectric thin-film filter 23a where thelight signal (λ₃) is reflected and travels in the reverse directionthrough the glass spacer 60 and the dielectric thin-film filter 23b tobe focused on the end face 26 of the rod lens 22. The position on theend face 26 at which the reflected light signal is focused is deviatedfrom the position of the port of the optical fiber 9' by a distance d₁.The port of the optical fiber 7 is disposed at the position deviatedfrom the optical fiber 9' by d₁. To this end, the inclination angle θ₁of the dielectric thin-film filter 23a is so selected that the lightsignal of wavelength λ₃ originating in the LD 1" is reflected by thefilter 23a and focussed into the optical fiber 7. The light signal(wavelength λ₃) transmitted through the optical fiber 7 in the up-linkdirection indicated by the arrow 8 reaches the dielectric thin-filmfilter 25a after having propagated through the rod lens 24, whereby thelight signal (λ₃) is reflected by the filter 25a and propagates throughthe rod lens 24 in the reverse direction to be focused and coupled tothe optical fiber 35 whose entrance end is disposed on the end face 27of the lens 24 at the position at which the reflected signal light (λ₃)is focused. When the distance between the optical fibers 7 and 35 on theend face 27 of the rod lens 24 is represented by d₂, the followingrelation applies valid: ##EQU2##

The light signal of wavelength λ₃ transmitted through the optical fiber35 in the direction indicated by an arrow 36 is received by aphotoelectric transducer element 11" to be amplified by an amplifier 12"and demodulated to the information signal through a demodulator 13".

The laser light of wavelength λ₂ emitted by the LD 1' is collimated by aspherical lens 2' and transmitted through the dielectric thin-film 25b,a glass spacer 62 and the dielectric thin-film filter 25a to enter therod lens 24. The light signal having traveled through the rod lens 24 isfocused onto the optical fiber 7 to be transmitted therethrough in thedirection indicated by an arrow 8' and enter the rod lens 22. The lightsignal of wavelength λ₂ having propagated through the rod lens 22 isreflected by the dielectric thin-film filter 23b to travel through therod lens 22 in the reverse direction, being focused onto the opticalfiber 33. The light signal of wavelength λ₂ transmitted through theoptical fiber 33 in the direction indicated by an arrow 34 is receivedby a photoelectric transducer element 11' and amplified by an amplifier12' whose output signal is demodulated by a demodulator 13' to the dataor information signal.

It will be appreciated that the two-way optical transmission of four ormore light signals of different wavelengths can be readily realized in asimplified structure in the similar manner. According to the teaching ofthe invention embodied in the system illustrated in FIG. 7, a single rodlens is imparted with the function for coupling the laser light signalto the optical fiber and the optical demultiplexing (ormultiplexing/demultiplexing) function. Accordingly, the number of thecomponent devices or elements can be decreased (down to 1/5 or less whencompared with the prior art system), as the result of which theinsertion loss as well as margin loss due to error involved in theoptical axis positioning can be significantly reduced. Taking numericalexamples, loss in the hitherto known optical multiplexer/demultiplexerfor three wavelength signals is about 5 dB per piece. Since a pair ofoptical multiplexers/demultiplexers have to be installed at both ends ofthe optical fiber, respectively, the total loss in the opticalmultiplexers/demultiplexers amount to as great as about 10 dB. Incontrast, it has been found that in the case of the two-way transmittingsystem designed for three light signals, the total loss in thedielectric thin-film filters is about 1.5 dB while the loss due to theoptical axis positioning error is about 1.2 dB, the sum of losses beingabout 2.7 dB. Accordingly, the inventive system can be improved by 7.3dB in respect to the loss over the prior art system. By using the laserlight of the long wavelength band (longer than 1 μm), the length of theoptical fiber can be increased more than 10 km over the length of theoptical transmission fiber used in the hitherto known two-way opticaltransmission system. Additionally, it has been established that thelaser light coupling efficiency to the optical fiber remainssubstantially the same as that of the hitherto known system. Since thetwo-way transmission system according to the present invention can beimplemented in an integral two-way transmission LD module in which theLD, the spherical lens, the rod lens attached with the dielectricthin-film filter and the pig-tail optical fiber are integrally combined,the inventive optical transmission system can be realized in a miniaturesize and enjoy very high reliability. Further, the manufacturing costcan be reduced to less than 1/2 of the cost required in the hithertoknown system. Thus, the invention opens a way to practical applicationof the optical communication system in the near future.

The angles θ₁ and θ₂ may be selected from a range of a fraction tothirties degrees. By increasing the angle θ, the distance among theoptical fiber port portions juxtaposed to one another on the end face(26, 27) of the rod lens can be correspondingly enlarged. Although thesystem shown in FIG. 7 is of a substantially symmetric circuitconfiguration, the invention is not restricted to such arrangement. Inthe case of the single-mode optical fiber transmission system in whichthe single-mode optical fiber is used as the optical fiber 7, theoptical fiber 9' is also constituted by the single-mode optical fiber.When the lengths of the optical fibers 9, 33 and 35 are sufficientlyshort, the multi-mode optical fiber having a large core diameter andsignificant difference in the refractive index can be employed as theseoptical fibers. Further, the photoelectric transducer element may beattached to the associated end face (26, 27) of the rod lens (with orwithout interposition of a lens). In that case, the optical fibers 9, 33and 35 may be eliminated. The invention can be equally applied to thetwo-way transmission system in which the optical fiber 7 is constitutedby a multi-mode optical fiber. In that case, the spherical lensinterposed between the LD and the rod lens may be spared. In the systemshown in FIG. 7, the distance between the end face of the LD and thespherical lens is selected equal to the focal length of the sphericallens, while the distance between the spherical lens and the rod lens isselectd equal to the sum of the focal lengths of these lenses. The focallengths of the spherical lens and the rod lens are selected independence on the spot size of the optical fiber and the diameter oflight beam emitted by the laser diode or LD.

FIG. 9 shows a two-way optical transmission system according to stillanother embodiment of the invention. In this optical transmissionsystem, the optical fiber 7 is disposed on and along the center axes ofthe rod lenses 22 and 24, wherein other optical fibers 9' and 33 (or 9and 35) are disposed, respectively, above and below the optical fiber 7in juxtaposition to one another. In this conjunction, both surfaces ofthe glass spacer 60 (or 62) are formed with the dielectric thin-filmfilters 23a and 23b (or 25a and 25b), respectively, at a same angle θ₂of inclination. Operation of this optical transmission system is thesame as that of the system shown in FIG. 7. Accordingly, any furtherdescription will be unnecessary.

FIGS. 10A to 10D are views for illustrating on an enlarged scale thestructure exhibiting the coupling function for introducing the lightemitted by the LD shown in FIG. 7 to the optical fiber and the opticalmultiplexing or demultiplexing function. In the case of the structureshown in FIG. 10A, one end face of the rod lens 22 is attached with astack including a glass spacer 64, a dielectric thin-film filter 23b, aglass spacer 60 and a dielectric thin-film filter 23a in this order asviewed from the side of the rod lens. For positioning the dielectricthin-film filters 23a and 23b at a desired angle relative to theincidence plane of laser light, both end faces of each of the glassspacers 60 and 64 are ground at a corresponding angle of inclination. Inthe case of the structure shown in FIG. 10B, glass spacers 65 and 66 arejoined together, wherein the dielectric thin-film filters 23a and 23bare attached to the exposed surfaces of the spacers 65 and 66,respectively. The laminated assembly thus realized is then bonded to theone end face of the rod lens 22. The glass spacers 65 and 66 are ofsymmetrical configuration. The one end face of the rod lens 22 is groundobliquely or bevelled. In the structure shown in FIG. 10C, there isprovided a space 67 between the glass spacer/dielectric filter assembly(23a, 23b, 60, 64) and the rod lens 22 for the purpose of allowing thefine adjustment of the condition for focusing the laser light onto theoptical fiber. In the case of the structure shown in FIG. 10D,additional dielectric thin-film filters 68 and 69 are provided on theend face 26 of the rod lens 22 in abutting relation to the ends of theoptical fibers 9' and 33, respectively, to thereby assure more positiveisolation between the different wavelengths. More specifically, thefilter 68 is constituted by a band pass filter which passes only thelight signal of wavelength λ₂, while the filter 69 is constituted by aband pass filter capable of passing only the light of wavelength λ₃.With this structure, the incidence of the unwanted wavelengths to theassociated optical fibers can be effectively suppressed.

It should be mentioned that the invention is not restricted to thesystem arrangements described above. For example, the spherical lenses2, 2' and 2" may be replaced by cylindrical lenses without departingfrom the concept of the invention. Further, the number of wavelengths asmultiplexed is not limited to three. For example, one end face of eachrod lens may be attached with three or more dielectric thin-film filtersat different angles of inclination, wherein four or more optical fibersmay be disposed on the side of the other end face of the rod lens.Alternatively, in the optical transmission system shown in FIG. 7 or 9,the end face of the rod lens 29 on which the laser light impinges may beattached with a dielectric thin-film filter in the manner similar to therod lens 22, whereby one more optical fiber can be provided adjacent tothe other end face of the rod lens (refer to FIG. 4). In that case, arod lens attached with a dielectric thin-film filter may be disposed infront of the photoelectric transducer 11 of the receiver station.

For packaging the LD and the spherical lens, they are conventionallyhoused in a container or receptacle having an optical or light window inan airtight manner, wherein the laser light is taken out through thewindow. In carrying out the invention, the similar packaging techniquecan be adopted.

It should be mentioned that a light emission diode (LED) may be used asthe semiconductor light emission element in place of the laser diode(LD). Since the LED has a broader spectral distribution than the LD, itis desirable to use the dielectric thin-film filter having steeperdamping or attenuating characteristic and increase the wavelengthspacing. In this manner, the two-way optical transmission which has beenheretofore realized by using two optical transmission fibers can berealized by using only one optical transmission fiber, promising thecost reduction.

The length of the rod lens may be slightly shorter or longer than n/4pitch (where n is 1, 3, 5 . . . ) in view of the addition of thedielectric thin-film filters and the glass spacers.

It goes without saying that the two-way optical transmission system canbe adopted not only for digital transmission but also for analoguetransmission. Further, in the systems shown in FIGS. 7 and 9, the endfaces 26 and 27 of the rod lenses 22 and 24 may be cut or ground slantat an angle θ_(r) (not shown) in a range of a fraction to tens degreesas shown in FIG. 11A and/or applied with an anti-reflection coating (ARcoating) 701 as illustrated in FIG. 11B, with a view to suppressingpossible leakage or crosstalk and interference of undesired opticalsignals to the desired optical signal at the individual receivers. Inthat case, when the angle θ_(r) is selected, for example, at 8 degrees,attenuation due to the reflection at the end face of the rod lens inconcern amounts to about 35 dB. Further, the attenuated optical signalis again attenuated more than 10 dB due to reflection by the dielectricthin-film filter and enters the optical fiber, being accompanied withthe desired coupling loss, whereby near-end crosstalk attenuation ofmore than 50 dB can be attained. Additionally, the ends of the opticalfibers disposed at the exit end of the rod lens in concern may be groundslant at the desired angle mentioned above, as is shown in FIGS. 11A to11C, to thereby increase the crosstalk attenuation, which may be furtherincreased by combining the anti-reflection coating 701 with thedielectric thin-film filters 68 and 69. In other words, the dielectricthin-film filters 68 and 69 are disposed on the exit end face of the rodlens 22 in opposition to the ends of the optical fibers 9' and 33 forpreventing a part of light emitted by the LD 1 from entering the opticalfibers 9' and 33 due to reflection at the exit end face of the rod lens22. Needless to say, the characteristics of these dielectric thin-filmfilters 68 and 69 are so designed as to attenuate the light produced bythe laser diode or LD 1.

It is noted that the provision of these dielectric thin-film filters 68and 69 are effective to suppress the crosstalk due to the spontaneouslight emission of the LD (or LED). In particular, this feature isimpotant for the optical analogue transmission.

The dielectric thin-film filters are not restricted to the opticalcharacteristics described above in conjunction with the illustratedembodiments. The band pass filter, short-wave band pass filter andlong-wave band pass filters can be employed in various combinations.

As will be appreciated, by forming the dielectric thin-film filter onthe rod lens for introducing light emitted by the semiconductor lightemission element to the optical fiber according to the teaching of theinvention, the optical coupling function and the opticalmultiplexing/demultiplexing function can be realized without requiringthe provision of the hitherto known optical multiplexer of a complicatedstructure constituted by a large number of components. In this way, thetwo-way optical transmission system characterized by low loss andextremely simplified structure can be realized, which is suited for along distance transmission.

FIG. 12 is a block diagram showing a general arrangement of a two-wayoptical transmission system according to a further embodiment of thepresent invention. The system shown in FIG. 12 differs from the oneshown in FIG. 9 in that optical fiber holding means or holders 70 and 71are provided at locations adjacent to the light exit end faces of therod lenses 22 and 24, respectively. The structure of the optical fiberholders 70, 71 will be described in detail hereinafter.

In operation, upon application of an information or data signal to theinput terminal 15, the LD drive circuit 14 operates to drive the LD 1.The laser light of wavelength λ₁ emitted by the LD 1 is collimated bythe spherical lens 2 and passes through the dielectric thin-film filter23a, the glass spacer 60, the dielectric thin-film filter 23b and therod lens 22 to enter the optical fiber holder 70. In this conjunction,when the rod lens 22 and the optical fiber holder 70 are so positionedthat the center axes of them practically coincide with each other withthe optical fiber 7 being disposed on the center axis, the adjustmentfor maximizing the light signal entering the optical fiber can beeffected merely by moving the optical fiber holder 70 in the axialdirection as indicated by an arrow 72.

Through the adjustment mentioned above, the laser light of wavelength λ₁is focused into the optical transmission fiber 7 held within the opticalfiber holder 70 to be transmitted through the optical fiber 7 in thedirection indicated by the arrow 8 and enter the rod lens 24 thorugh theoptical fiber holder 71. The optical fiber 7 is disposed substantiallyon the center axis of the axially aligned optical fiber holder 71 androd lens 24, wherein the adjustment of the optical couplng between theoptical fiber holder 71 and the rod lens 24 can be accomplished simplyby moving the optical fiber holder 71 in the direction indicated by anarrow 73. The light signal of wavelength λ₁ having traveled through therod lens 24 impinges on the dielectric thin-film filter 25b through thedielectric thin-film filter 25a and a glass spacer 62. The light signalof wavelength λ₁ is reflected by the dielectric thin-film filter 25b andcaused to travel through the glass spacer 62, the dielectric thin-filmfilter 25a and the rod lens 24 in the reverse direction. Since thedielectric thin film 25b is formed on the end face of the rod lensslanted at an angle θ₁ ', the position on the end face 27 of the rodlens 24 at which the reflected light is focused is deviated from theentrance or incidence position by a distance proportional to the angleθ₁ '. The position of the optical fiber 9 in the optical fiber holder 71is previously so determined that the fiber 9 can be disposed at theposition where the reflected light signal is focused. Then, theadjustment of the optical coupling can be readily accomplished simply byrotating the optical fiber holder 71 in the circumferential direction,as indicated by an arrow 75. The reflected light signal of wavelength λ₁then travels through the core of the optical fiber 9 in the directionindicated by an arrow 10 to be received by the photoelectric transducer11 and converted into an electric signal which is demodulated to theoriginal information signal by the demodulator 13 after having beenamplified by the amplifier 12.

Upon application of an information signal to the other input terminal15" of the up-link circuit, the LD drive circuit 14" drives the LD 1"which oscillates at a wavelength λ₃ to emit the laser light of thewavelength λ₃ which is then collimated by the spherical lens 2" to enterthe rod lens 32. The light signal propagated through the rod lens 32 isfocused into the optical fiber 9' to travel therethrough in thedirection indicated by an arrow 10' and enter the rod lens 22 by way ofthe optical fiber holder 70. The light signal of wavelength λ₃ enteringthe rod lens 22 reaches the dielectric thin-film filter 23a after havingpassed through the dielectric thin-film filter 23b and the glass spacer60. The light signal of wavelength λ₃ is reflected by the filter 23a totravel in the reverse direction through the glass spacer 60, thedielectric thin-film filter 23b and the rod lens 22 to be focused intothe core of the optical transmission fiber 7 mounted in the opticalfiber holder 70 to be transmitted through the fiber 7 in the directionindicated by the arrow 8. The angle θ₁ is previously so selected thatthe light signal of wavelength λ₃ is reflected by the dielectricthin-film filter 23a to be thereby focused into the optical transmissionfiber 7 mounted in the optical fiber holder 70. The light signal ofwavelength λ₃ transmitted through the optical fiber 7 in the directionindicated by the arrow 8 enters the rod lens 24 by way of the opticalfiber holder 71 to reach the dielectric thin-film filter 25a. The lightsignal of wavelength λ₃ is reflected by this dielectric thin-film filter25a to be transmitted through the rod lens 24 in the reverse directionand focused into the core of the optical fiber 35 held by the opticalfiber holder 71.

The angle θ₂ ' is previously set at such a value that the light signalof wavelength λ₃ reflected at the dielectric thin-film filter 25a can befocused to the optical fiber 35 mounted in the optical fiber holder 71.The light signal transmitted through the optical fiber 35 in thedirection indicated by an arrow 36 is received by the photoelectrictransducer element 11" whose electrical output signal is amplified bythe amplifier 12" and subsequently demodulated to the originalinformation signal by the demodulator 13".

Upon application of an information signal to the input terminal 15' ofthe down-link circuit, the LD drive circuit 14' drives the LD 1' whichthen emits a laser light signal of wavelength λ₂. This light signal iscollimated by the spherical lens 2' and enters the rod lens 24 afterhaving transmitted through the dielectric thin-film filter 25b, theglass spacer 62 and the dielectric thin-film filter 25a. The lightsignal of wavelength λ₂ having traveled through the rod lens 24 isfocused to the optical transmission fiber 7 mounted in the optical fiberholder 71 to be transmitted through the fiber 7 in the directionindicated by the arrow 8' and enter the rod lens 22 by way of theoptical fiber holder 70. The light signal of wavelength λ₂ havingtraveled through the rod lens is reflected by the dielectric thin-filmfilter 23b and caused to travel through the rod lens 22 in the reversedirection, to be thereby focused into the optical fiber 33. The lightsignal of wavelength λ₂ transmitted through the optical fiber 33 in thedirection indicated by the arrow 34 is received by the photoelectrictransducer element 11' to be converted into a corresponding electricsignal which is then amplified by the amplifier 12' and demodulated tothe original information signal by the demodulator 13'. It should benoted that the reflecting end faces 26 and 27 of the rod lenses 22 and24 as well as both end faces of the rod lens 32 are ground slant for thepurpose of protecting the LD and/or the photoelectric transducer fromthe influence of the reflected light signals. Each of these end facesmay be additionally applied with an anti-reflection coating (ARcoating).

As will be apparent from the foregoing description, in the case of thetwo-way optical transmission system shown in FIG. 12, there are disposedon the entrance end face of the rod lens 22 which serves to couple thelaser light emitted by the LD 1 to the optical transmission fiber 7 oneor more dielectric thin-film filters 23a and 23b which havepredetermined optical characteristics, respectively, while disposed onthe side of the exit end face of the rod lens 22 is the optical fiberholder 70 which serves for positioning and holding fixedly in anintegral manner the optical transmission fiber 7 substantially on thecenter axis of the rod lens 32, the branching optical fiber 33 fortransmitting the light signal separated by the dielectric thin-filmfilters 23a and 23b located on the side of the optical transmissionfiber 7 and additionally the multiplexing optical fiber 9' which servesto introduce the light emitted by the other LD 1" of differentoscillation wavelength to the dielectric thin-film filters 23a and 23bfor multiplexed transmission. The rod lens 22 and the optical fiberholder 70 disposed fixedly and integrally constitute an optical modulefor the two-way optical transmission.

Next, description will be made on the structure of the optical fiberholders 70 and 71.

FIGS. 13A and 13B are views showing a structure of the optical fiberholder 70 (or 71) which can be employed in the transmission system shownin FIG. 12, wherein FIG. 13A is a front view and FIG. 13B is an axialsectional view.

Referring to the figures, the optical fiber holder 70 (or 71) has acylindrical structure provided with through-holes 43, 44 and 45 in whichthe optical fibers are inserted, respectively. As will be appreciatedfrom the description in conjunction with FIG. 12, the through-hole orbore 43 is destined to receive the optical transmission fiber 7, thehole 44 is destined to mount therein the optical fiber 9' (or 9), andthe hole 45 is destined to mount the optical fiber 33 (or 35). Thedistance between the through-holes 43 and 44 and the distance betweenthe through-holes 43 and 45 are determined in dependence on the anglesθ₁ (or θ₁ ') and θ₂ (or θ₂ ') of the dielectric thin-film filters 23aand 23b (or 25a and 25b) provided on the slanted end face of the rodlens 22 (or 24), respectively, and are in proportion to these angles. Onthe other hand, the distance in concern is in reverse proportion to theproduct of the refractive index and the index gradation constant of therod lens 22 (24). As will be seen in FIG. 13A, the through-hole 43 isformed on the center axis of the cylindrical holder 70 (71) (whichpasses through the intersection of the X--X' axis and the Y--Y' axis).As will be seen in FIG. 13B, the holder 70 (71) has an end face oppositeto the rod lens 22 (24) which is bevelled at an angle θ₄, while theother end of the optical fiber holder is formed with a cylindricalrecess of a large diameter for accommodating a number of the opticalfibers therein.

FIGS. 14A and 14B are views similar to FIGS. 13A and 13B and show theoptical fiber holder 70 (71) in the state in which the optical fibersare mounted.

The one end face 46 of the optical fiber holder 70 (71) (which is inopposition to the rod lens) inclusive of the end faces of the opticalfibers is ground or bevelled at an angle θ₄. This is for the purpose ofpreventing the light signal from being reflected at the ends of theoptical fibers to enter again the LD, the phototelectric transducer orthe optical fibers. The angle θ₄ should preferably be set in a range ofa fraction to 10 degrees. Usually, the angle θ₄ is slected equal toabout 8 degrees. This end face 46 may be applied with an anti-reflectioncoating (AR coating). For effecting the optical adjustment, the opticalfiber holder 70 (71) may be simply rotated in either one of thedirections indicated by arrows 74 and 74'. Among the three opticalfibers 7, 9' and 33 fixedly inserted and mounted in the optical fiberholder, the optical transmission fiber 7 is constituted by a single-modeoptical fiber, while the other two fibers 9' and 33 are constituted bymulti-mode optical fibers, respectively.

FIG. 15A and FIG. 15B show in a front view and a longitudinal sectionalview of another structure of the optical fiber holder which may be usedin the optical transmission system shown in FIG. 12. This holder isdesigned to mount fixedly two optical fibers 7 and 33 in V-like grooves77 and 78, respectively. As will be seen in FIG. 15A, the optical fiberholder is composed of a pair of semicylindrical members 47 and 48 formedwith the V-like grooves 77 and 78, respectively. After the opticalfibers 7 and 33 have been fixedly disposed in the respective V-likegrooves, the semicylindrical members 47 and 48 are joined together toconstitute the optical fiber holder 70 of the cylindrical configuration.In this case, the optical fiber 7 is required to be disposed along thelongitudinal axis (i.e. the axis extending perpendicularly to the X--X'axis and the Y--Y' axis at the intersection). Accordingly, thesemicylindrical members 47 and 48 are not divided along the planecontaining the center axis but divided along the plane containing thetop of the V-like groove 77 formed in the semicylindrical member 47.

FIGS. 16A and 16B show a version of the optical fiber holder shown inFIGS. 15A and 15B, which is designed to hold three optical fibers in theV-like grooves.

In this case, the optical transmission fiber 7 is stationarily fixed atthe center coinciding with the intersection between the X--X' axis andthe Y--Y' axis. It should be noted that optical fibers having large corediameter and outer diameter are used as the optical fibers 9' and 33with an attempt to increase the light signals supplied to the associatedphotoelectric transducers and compensate for the misalignment of theoptical axes.

The distance between the optical fibers 7 and 9' and the distancebetween the optical fibers 7 and 33 are determined in dependence on theangles θ₁ and θ₂ at which the dielectric thin-film filters 23a and 23bare disposed (refer to FIG. 12). Deviations of the distance between theoptical fibers 7 and 9' and between the optical fibers 7 and 33 due tomanufacturing errors involved in the angles θ₁ and θ₂ can besufficiently compensated for by using the optical fibers 9' and 33 of anincreased core diameter. Accordingly, the optical coupling can bereadily adjusted merely by rotating the optical fiber holder 70 in thecircumferential direction 74 or 74', as shown in FIG. 14B. The structureshown in FIGS. 16A and 16B differs from the one shown in FIGS. 15A and15B in that the cylindrical optical fiber holder is constituted by twosemicylindrical halves 49 and 50 divided along the vertical center planeY--Y', each of the cylindrical halves being formed with three V-likegrooves. It will be seen that the lateral V-like grooves are formed inlarger size than the center V-like groove. This is because the opticalfibers 9' and 33 of larger core diameter and outer diameter are disposedabove and below the central optical fiber 7 as viewed in the verticaldirection. It should be mentioned that in the case of the holderstructures illustrated in FIGS. 14A, 14B, 15A and 15B, positionaldeviations due to the manufacturing error can be compensated for byusing the optical fibers having a large core diameter and outerdiameter.

FIG. 17 is a longitudinal sectional view showing an integrated modulestructure which comprises a laser diode or LD, a glass rod forextracting a light signal for the monitoring purpose, a spherical lens,a rod lens attached with dielectric thin-film filters and an opticalfiber holder. Referring to FIG. 17, a reference numeral 50' denotes themonitor light extracting glass rod, and 51, 52 and 54 denote parts forpackaging, 55 denotes an optical window, and 56 denotes an electrode.The laser diode or LD 1 is mounted at a position adjacent to theelectrode 56 with the spherical lens 2 being mounted adjacent to theLD 1. The end face of the rod lens 22 located on the side of thespherical lens 2 is attached with the dielectric thin-film filters 23aand 23b, wherein the optical window 55 is interposed between the filters(23a, 23b) and the spherical lens 2. Disposed on the side of the otherend face of the rod lens 22 is the optical fiber holder 70 with a smallgap relative to the lens 22, wherein three optical fibers 7, 9' and 33are inserted stationarily in the holder 70.

As will be seen in FIG. 17, when the optical fiber holder 70 (71) of acylindrical configuration is used, the LD 1, the monitor lightextracting glass rod 50', the spherical lens 2, the rod lens 22 equippedwith the dielectric thin-film filters and the optical fiber holder 70can be assembled in an integral structure, whereby the optical modulefor the two-way transmission can be realized. A photoelectric transducerelement for the monitoring may be disposed immediately downstream of theLD (on the side opposite to the spherical lens) or alternativelydownstream of the glass rod 50'. With this arrangement, significantlyhigh reliability, miniaturization, inexpensiveness and loss reductioncan be accomplished, as compared with the hitherto known opticaltransmission system in which the optical multiplexer is discretelyemployed.

It should be mentioned that light emission diode of end-face emissiontype or planer emission type may be used as the laser oscillator inplace of the laser diode. In case the light emission diode is employed,it is preferred that an optical focusing system including a lens, aparabolic element or the like may be disposed at the emission side ofthe light emission diode or LED to thereby collimate the light emittedby the LED before it enters the rod lens equipped with the dielectricthin-film filters. Since the LED has a broader spectral distributionthan the LD, it is desirable to use the dielectric thin-film filtersimparted with steeper attenuation characteristics and having greaterwavelength spacing. Further, the entrance end of the optical fiber 7 maybe configured in a spherical form or alternatively attached with aspherical lens.

With the structure of the optical fiber holder in which the opticaltransmission fiber is fixedly disposed substantially on the longitudinalcenter axis of the holder while the demultiplexing optical fiber and themultiplexing optical fiber are fixedly juxtaposed to the opticaltransmission fiber with a predetermined distance therebetween, theadjustment of optical coupling to the optical transmission fiber can beeffected simply by rotating the optical fiber holder. Since theadjustment of optical coupling to the optical transmission fiber can becarried out independent of the adjustment of the optical coupling to thedemultiplexing optical fiber and the multiplexing optical fiber, theoptical adjustment is facilitated, whereby skillfulness is no morerequired. Further, the optical fiber holder of the cylindrical ofcircular column-like configuration is insensitive to variations intemperature. By virtue of the arrangement such that the optical fibersare integrally secured to the optical fiber holder of cylindrical orcolumn-like configuration, the united structure exhibits significantresistance or enhanced withstanding capability to vibration or shock,assuring that the distance or spacing among the individual opticalfibers is not susceptible to variations. In view of the fact that theinter-fiber distance is on the order of several hundred microns, theseparate mounting of the individual optical fibers is disadvantageous,because the positions of the optical fibers will be likely to undergovariations under vibration. Besides, the mounting or fixing of theindividual fibers separately is very troublesome and difficult to berealized. Further, by securing fixedly the rod lens and the cylindricalor column-like optical fiber holder by using a separately preparedcylindrical container in the form of an integral unit, the stability ofthe integral unit against the temperature changes and vibration can bemuch enhanced.

Further, since the two-way optical transmission system is imparted withthe two functions, i.e. the optical coupling function and the opticalmultiplexing and demultiplexing functions, by using the rod lenses eachprovided with the dielectric thin-film filter for introducing lightsignals emitted by the semiconductor light emission element (LD, LED andthe like), the two-way transmission system according to the inventioncan be implemented in a much simplified structure, whereby the insertionloss as well as manufacturing cost can be significantly reduced ascompared with the hitherto known system. In this connection, it shouldbe recalled that the insertion loss of the opticalmultiplexer/demultiplexer is about 5 dB per piece in the case of theprior art transmission system. In contrast, in the illustrated systemsembodied according to the invention, the loss in concern is reduced toone-third or less, whereby the transmission distance can becorrespondingly increased. The invention can be applied effectivelyparticularly to the optical two-way transmission system in which theoptical transmission line is constituted by a single-mode optical fiber.

The dielectric thin-film filters having optical characteristics(transmission and reflection characteristics) which differ from one toanother filters may be formed on the LD-light entrance end face of therod lens in n layers at mutually different angles θ_(n) (n=1, 2, 3 . . .). By bevelling the other end face of the rod lens and disposing theoptical transmission fiber substantially on the center axis of the rodlens with n demultiplexing and multiplexing optical fibers beingdisposed around the optical transmission fiber, all the fibers beingsecured by the optical fiber holder, it is possible to realize theoptical transmission of n wavelengths by using a pair of the opticalmodules. The end face portions of the rod lens which are located inopposition to the demultiplexing optical fiber and the multiplexingoptical fiber may be formed with the dielectric thin-film filters,respectively, for suppressing the near-end crosstalk and allowing onlythe light of desired wavelength to pass therethrough.

It should be added that the optical fiber holder is not restricted tothe cyclindrical or circular column-like configuration. A prism-likeconfiguration or other may be adopted on the condition that the opticaltransmission fiber (the fiber 7 in the illustrated system) can bepositioned substantially along the center axis.

As will be appreciated from the foregoing, by virtue of the muchsimplified structure of the two-way optical transmission module whichalso allows the facilitated adjustment of the optical coupling, the timetaken for the adjustment of the optical coupling can be significantlyreduced, while the high-precision coupling adjustment as well as thecost and loss reduction can be assured. Thus, the system according tothe invention promises practical realization of the long-distanceoptical transmission system. It is further to be noted that the two-wayoptical transmission module according to the invention exhibits highstability and reliability even under adverse environmental conditions.

FIG. 18A to FIG. 24B show other embodiments of the two-way opticaltransmission module according to the invention.

FIGS. 18A to 18D illustrate stepwise a process of manufacturing a rodlens assembly, according to which a rod lens is inserted and secured ina cylindrical tube having a reference mark, being followed by a step ofgrinding slant the end faces of the assembled rod lens, which faces arethen formed with dielectric thin-film filters. In FIGS. 18A to 18D,front views are shown at (A), right side views are shown at (B) and leftside views are shown at (C). At a step shown in FIG. 18A, the outerperiphery of a rod lens 80 is coated with a thin film 81 (metal film orbonding agent film). At a step shown in FIG. 18B, the rod is insertedand secured in a cylindrical tube 82 having a protrusion 83 serving as amark. The cylindrical tube may be made of glass, ceramic or metalmaterial. The rod lens can be secured by using a bonding agent or bywelding or soldering. At a step shown in FIG. 18C, the rod assembly isso positioned that the mark protrusion 83 lies on the Y--Y' axis andsubsequently both end faces of the rod lens assembly are ground slant atdesired angles θ₁ and θ₃, respectively. At a step illustrated in FIG.18D, one of the end faces is formed with a dielectric thin-film filter84 while the other end face is formed with an aniti-reflection film 85for suppressing reflection.

FIGS. 19A and 19B show an exemplary embodiment of the two-way opticaltransmission module which incorporates the rod lens provided with theenclosing tube shown in FIGS. 18A and 18D. In the figure, a referencenumeral 86 denotes the rod lens fixedly accommodated within thecylindrical tube as shown in FIG. 18D, and 87 denotes a receiving basewhich is provided with a recess 89 for fitting therein the encased rodlens. In other words, the receiving base 86 is a female menber, so tosay, while the encased rod lens constitutes a male member, whereby theassembling or packaging of both the members can be effectively carriedout within a short time, advantageously for the manufacture on the massproduction basis. A reference numeral 90 denotes a glass rod forderiving light signal for the purpose of monitoring. Numerals 91, 92 and93 denote parts which constitute the package, 94 denotes an opticalwindow and 96 denotes an electrode.

FIGS. 20A and 20B and FIGS. 21A and 21B show other embodiments of therod lens encased within the cylindrical tube and the receiving base.More specifically, FIGS. 20A and 21A show end views of the tube-encasedrod lenses, and FIGS. 20B and 21B show end views of the receiving basesor members. In the case of the embodiment illustrated in FIGS. 20A and20B, flattened portions 97 serving for the positioning reference markare formed in the cylindrical tube 98, while the receiving member 87 isformed with a recess or bore 99 which is adapted to receive snugly thecylindrical tube 98. On the other hand, in the case of the embodimentillustrated in FIGS. 21A and 21B, the cylindrical tube 101 is providedwith a pair of diameterically opposite grooves 100 serving for thepositioning mark, while the receiving base or member 87 is formed with abore 102 of the same shape as the outer profile of the cylindrical tube101.

FIGS. 22A and 22B show another embodiment of the invention in which aprotrusion 83 serving for the positioning mark is formed in a holder 105used for securing the optical fibers 7 and 9'. The outer configurationof the holder 105 is so realized as to be snugly engaged in the bore 89of the receiving base 87 shown in FIGS. 19A and 19B. A reference numeral104 denotes an anti-reflection coating film. Since the optical fiber 9'is fixedly disposed on the Y--Y' axis, the light signal emitted by thesemiconductor light emission element 1 can be coupled to the opticalfiber 7 with an improved efficiency, while the light signal produced bythe semiconductor light emission element 1" can be effectively coupledto the optical fiber 9', both couplings being realized in a facilitatedmanner.

FIG. 23 shows a two-way optical transmission module incorporating thetube-encased rod lens 110 shown in FIG. 18D and the optical fiber holder112 according to still another embodiment of the present invention.Bacause the guiding mechanism is available, the optical coupling can befinely adjusted by moving the rod lens 110 and the holder 112 slightlyin either direction Z or Z'. This structure also allows the timerequired for assembling to be shortened, while assuring cost reduction.This structure is also suited for the manufacturing on the massproduction basis.

FIGS. 24A and 24B show another embodiment of the invention according towhich the positioning marks are formed on the end faces of the rod lens.Referring to FIG. 24A, when the end faces of the rod lens 80 are groundslant along the Y--Y' axis at the desired angles θ₁ and θ₃,respectively, parts 114 and 115 are left unground to constitute thepositioning reference marks. These portions 114 and 115 are selected atthe regions of the associated end faces of the rod lens which areexcluded from illumination by the light beam emitted by thesemiconductor light emission element. The area of the mark may be about10% of the whole area of the end face. Referring to FIG. 24B, these endfaces are provided with a dielectric thin-film filter 84 and ananti-reflection film 85, respectively. By preparing the positioningmarks in this way, the rod lens can be readily disposed at the desiredposition in the course of packaging.

FIG. 25 shows a two-way optical transmission system according to stillanother embodiment of the present invention. Referring to FIG. 25,optical transmitters 151 to 153 and an optical receiver 134 are disposedat one end of a single optical fiber 7 while an optical transmitter 154and optical receivers 131 to 133 are disposed at the other or oppositeend of the optical fiber 7. Each of the optical transmitters and theoptical receivers is combined with an associated dielectric thin-filmfilter 201, . . . , or 208 and rod lens 301, . . . , or 308 toconstitute a combined unit. One end of the optical fiber 7 is coupledwith one of the combined units (the one including the opticaltransmitter 151 in the case of the illustrated embodiment) disposed atsaid one end of the optical fiber 7, while the other end thereof iscoupled to a given one of the combined units (the one including theoptical receiver 131 in the case of the illustrated embodiment) whichare disposed on the side of said other end of the optical fiber 7. Therod lenses disposed at one end of the single optical fiber 7 areconnected to cascade through optical fibers 401 to 404, and those rodlenses which are disposed at the other end of the single optical fiberare also connected in cascade through optical fibers 405 to 408.

The individual thin-film filters are imparted with such characteristicsthat they transmit therethrough the light signals of wavelengths whichare emitted by the associated or combined optical transmitter or whichare to be received by the associated optical receivers while reflectingthe other irrelevant light signals. Accordingly, the light signal ofwavelength λ₁ takes the course of 151-201-301-7-305-205-131. Similarly,the light signal of wavelength λ₂ follows the course of152-203-303-402-302-202-302-401-301-201-301-7-305-205-305-405-306-206-306-406-307-207-132.The light signal of wavelength λ_(n) follows the course of153-204-304-404-403-303-203-303-402-302-202-302-401-301-201-301-7-305-205-305-405-306-206-306-406-307-207-307-407-408-308-208-133.The light signal of wavelength λ_(m) follows the course of154-206-306-405-305-205-305-7-301-201-301-401-302-202-134.

One of the characteristic features of the illustrated embodiment residesin that no restriction is imposed at all on the order of arraying theoptical transmitters and the optical receivers disposed at both ends ofthe optical fiber 7, respectively. For example, the combined unitincluding the optical transmitter 151 can be replaced by the combinedunit including the optical receiver 134 with regard to the disposedlocation. In the similar manner, the combined units including theoptical transmitters or optical receivers, respectively, and disposed onthe same side of the single optical fiber can be interchanged with oneanother with respect to the disposed location. Of course, the pathsfollowed by the light signals of different wavelengths will becorrespondingly changed. However, no problem arises with regard to theessence of the two-way optical transmission. Rather, there will beobtained advantages mentioned below. That is, the individual combinedunits including the optical transmitters or optical receivers,respectively, can be realized in the module of a same structure exceptfor the dielectric thin-film filters which differ from one to anothermodule. Further, any given combined unit including an opticaltransmitter or an optical receiver designed for dealing with a givenwavelength can be added or removed merely by connecting or disconnectingthe optical fiber to or from the rod lens of the combined unit inconcern. In this manner, expansion or contraction of the system can beaccomplised in a facilitated manner.

The present invention is not restricted to the disclosed embodiments.The semiconductor light emission element may be either a semiconductorlaser or a light emission diode. The optical fiber may be of a singlemode optical fiber or a multi-mode optical fiber. In place of the rodlens, an indexgraded slab lens may be used. The invention may be appliedto the transmission of three or more light signals in addition to thetransmission of two light signals.

As will be appreciated, by providing the mark which indicates that thedielectric thin-film filter is formed on the end face of the rod lens ata desired angle along the Y--Y' axis, the optical coupling of the rodlens to two optical fibers can be easily accomplished, whereby the timerequired for the assembling as well as the manufacturing cost can besignificantly reduced. By providing the optical fiber holder and thereceiving base which are adapted to be packaged or assembled withreference to the mark, the adjustment of the optical axis isfacilitated. Further, the optical module is suited to be manufactured onthe mass-production basis, because of the simplified process of securingthe rod lens within a cylindrical tube provided with a mark, grindingslant the rod lens in the state encased within the cylindrical tube,forming the dielectric thin-film filter and finally packaging theassembly.

We claim:
 1. A two-way optical transmission system in which a pluralityof light signals of wavelengths differing one from another aretransmitted through a single optical fiber from one end thereof to theother end and from said other end to said one end, comprising:firstoptical transmitter means for converting first input information into afirst light signal of wavelength λ₁ and sending out said first lightsignal; second optical transmitter means for converting second inputinformation into a second light signal of wavelength λ₂ and outputtingsaid second light signal; a first combination of a first dielectricthin-film filter and a first rod lens interposed between said one end ofsaid single optical fiber and said first optical transmitter means, saidfirst dielectric thin-film filter transmitting therethrough said firstlight signal of wavelength λ₁ and reflecting said second light signal ofwavelength λ₂, said first rod lens having one end facing said one end ofsaid optical fiber; a second combination of a second dielectricthin-film filter and a second rod lens interpoed between said other endof said single optical fiber and said second optical transmitter means,said second dielectric thin-film filer transmitting therethrough saidsecond light signal of wavelength λ₂ and reflecting said first lightsignal of wavelength λ₁, said second rod lens having one end facing saidother end of said optical fiber; first optical receiver means forreceiving said first light signal of wavelength λ₁ reflected by saidsecond dielectric thin-film filter by way of the one end of said secondrod lens and converting said first light signal into said firstinformation; and second optical receiver means for receiving said secondlight signal of wavelength λ₂ reflected by said first dielectricthin-film filter by way of the one end of said first rod lens andconverting said second light signal into said second information,wherein each of said first and second transmitter means has a lightemission element and a spherical lens, each of said first and seconddielectric thin-film filters is provided with an angle at an end face ofthe corresponding rod lens which is located on the entrance side oflight emitted by the light emission element, the length of each of saidfirst and second rod lenses is about n/4 pitch (where n=1, 3, 5, . . .), and each of said first and second transmitter means and thecorresponding filter and rod lens are integrally assembled in a singlereceptacle to thereby constitute an optical module.
 2. A two-way opticaltransmission system according to claim 1, wherin the one end of saidsecond rod lens is connected with said first optical receiver meansthrough a first optical fiber, and the one end of said first rod lens isconnected with said second optical receiver means through a secondoptical fiber.
 3. A two-way optical transmission system according toclaim 1, wherein each of said first and second dielectric thin-filmfilters reflects a third light signal of wavelength λ₃, furthercomprising:third optical transmitter means for converting third inputinformation into said third light signal of wavelength λ₃ and outputtingsaid third light signal; a third combination of a third dielectricthin-film filter, a third rod lens and a third optical fiber, said thirdcombination being interposed between said third optical transmittermeans and the one end of said first rod lens, said third dielectricthin-film filter transmitting therethrough said third light signal ofwavelength λ₃ and reflecting said second light signal of wavelength λ₂,said third optical fiber having one end facing the one end of said firstrod lens and having the other end facing one end of said third rod lens,said second optical receiver means receiving said second light signal ofwavelength λ₂ reflected by said third dielectric thin-film filter by wayof said one end of said third rod lens; third optical receiver means forreceiving said third light signal of wavelength λ₃ and converting saidthird light signal into said third information; a fourth combination ofa fourth dielectric thin-film filter, a fourth rod lens and a fourthoptical fiber, said fourth combination being interposed between saidthird optical receiver means and the one end of said second rod lens,said fourth dielectric thin-film filter transmitting therethrough saidthird light signal of wavelength λ₃ and reflecting said second lightsignal of wavelength λ₂, said fourth optical fiber having one end facingthe one end of said second rod lens and having the other end facing oneend of said fourth rod lens, said first optical receiver means receivingsaid first light signal of wavelength λ₁ reflected by said fourthdielectric thin-film filter by way of the one end of said fourth rodlens.
 4. A two-way optical transmission system according to claim 3,wherein the one end of said fourth rod lens is connected with said firstoptical receiver means through said fourth optical fiber, and the oneend of said third rod lens is connected with said second opticalreceiver means through said third optical fiber.
 5. A two-way opticaltransmission system according to claim 1, 2, 3 or 4, wherein at leastone of said rod lenses is composed of two parts, and said dielectricthin-film filter associated with said one rod lens is disposed betweensaid two parts.
 6. A two-way optical transmission system according toclaims 1, 2, 3 or 4, wherein at least one of said dielectric thin-filmfilters is formed on a surfaceof a glass spacer.
 7. A two-way opticaltransmission system in which a plurality of light signals of wavelengthsdiffereing one from another are transmitted through a single opticalfiber from one end thereof to the other end and from said other end tosaid one end, comprising:at least two optical transmitter meansdisposed, respectively, on both ends of said single optical fiber fortransmitting light signals of mutually different wavelengths; at leasttwo optical receiver means disposed, respectively, on both ends of saidsingle optical fiber for receiving the light signals from said at leasttwo optical transmitter means; combinations each composed of adielectric thin-film filter and a rod lens and disposed, respectively,between the ends of said single optical fiber and said at least twooptical transmitter means, each of said dielectric thin-film filterstransmitting therethrough the light signal of the wavelength transmittedby the associated optical transmitter means and reflecting the lightsignal of the wavelength transmitted by the counterpart opticaltransmitter means, each of said rod lenses having one end facing the endof said single optical fiber, said optical receiver means receiving thelight signals reflected by said dielectric thin-film filters by way ofsaid one ends of said rod lenses, respectively wherein each of saidtransmitter means has a light emission element and a spherical lens,each of said dielectric thin-film filters is provided with an angle atan end face of the corresponding rod lens which is located on theentrance side of light emited by the light emission element, the lengthof each of said rod lenses is about n/4 pitch (where n=1, 3, 5 . . . ),and each of said transmitter means and the corresponding filter and rodlens are integrally assembled in a single receptacle to therebyconstitute an optical module.
 8. A two-way optical transmission systemin which three or more light signals of wavelengths differing one fromanother are transmitted through a single optical fiber from one endthereof to the other end and from said other end to said one end,comprising:at least two optical transmitter means disposed,respectively, on both ends of said single optical fiber for transmittingthe lightsignal of mutually different wavelenths, respectively;combinations each of a rod lens and n dielectric thin-film filters(where n≧2) an disposed, respectively, between the ends of said singleoptical fiber and said optical transmitter means, each of saiddielectric thin-film filters transmitting therethrough the light signalof a specific wavelength and reflecting the light signals of the otherwavelengths, each of said rod lenses having one end facing one end ofsaid single optical fiber; n optical fibers disposed at each of said oneends of said rod lenses and having respective one ends facing the oneend of each rod lens in corresponding relation with said n dielectricthin-film filters, respectively, while the other ends of a pair of saidn optical fibers being connected to optical receiver means for receivingthe optical signals of different wavelengths transmitted from said twooptical transmitter means, respectively, to (n-1) optical transmittermeans for transmitting wavelengths differing from the wavelengths fromsaid two optical transmitter means and to (n-1) optical receiver meansfor receiving the light signals of wavelengths transmitted from said(n-1) optical transmitter/means, wherein each of said transmitter meanshas a light emission element and a spherical lens, each of saiddielectric thin-film filters is provided with an angle at an end face ofthe corresponding rod lens which is located on the entrance side oflight emitted by the light emission element, the length of each of saidrod lenses is about n/4 pitch (where n=1, 3, 5, . . . ), and each ofsaid transmitter means and the corresponding filter and rod lens areintegrally assembled in a single receptacle to thereby constitute anoptical module.
 9. A two-way optical transmisson system according toclaim 8, wherein both ends of said single optical fiber are disposed onthe center axes of said rod lenses, respectively.
 10. A two-way opticaltransmission system according to claim 8, wherein at least one of saiddielectric thin-film filters is formed on a surface of a glass spacer.11. A two-way optical transmission system according to claim 10, whereina space is provided between said glass spacer and the associated one ofsaid rod lenses.
 12. A two-way optical transmission system according toclaim 8, 9, 10 or 11, wherein said n optical fibers have respective endfaces provided with the dielectric thin-film filters which pass thespecific wavelengths, respectively.
 13. A two-way optical transmissionsystem according to claim 8, 9, 10 or 11, wherein each of said rodlenses has one end face bevelled at a predetermined angle relative to aplane perpendicular to the axial direction of said rod lens.
 14. Atwo-way optical transmission system according to claim 13, wherein theone end face of said rod lens bevelled at said predetermined angle isapplied with an anti-reflection coating.
 15. A two-way opticaltransmission system according to any one of preceding claims 2 to 4 and8 to 11, wherein optical fiber holders are disposed between said rodlenses and a plurality of said optical fibers facing said rod lenses.16. A two-way optical transmission system according to claim 15, whereina face of said optical fiber holder located on the side of said rod lensis bevelled at a predetermined angle relative to a plane perpendicularto the axial direction of said rod lens.
 17. A two-way opticaltransmission system according to claim 15, wherein each of said opticaltransmitter means constitutes an optical module comprising at least oneof said dielectric thin-film filters, rod lenses and optical fiberholders which are integrally assembled in a single receptacle, tothereby constitute an optical module for the two-way opticaltransmission, and wherein said optical module has a rod of glass formonitoring light of a light emission element included in said opticaltransmitter means.
 18. An optical module for two-way transmission,comprising:an electrode for receiving an input electrical signalsupplied externally; a light emission element connected to saidelectrode and converting said electric signal into a light signal; lensfor collimating the light signal emitted by said light emission element;an optical window for transmitting therethrough the light signal fromsaid lens; at least one dielectric thin-film filter for transmitting telight signal from said optical window; a rod lens for sending out thelight signal transmitted through said dielectric thin-film filter to anoptical fiber; a receiving base for securing said rod lens and having anaperture for receiving therein an optical fiber holder; and wherein saidat least one dielectric thin-film filter is provided with an angle at anend face of the corresponding rod lens which is located on the entranceside of said light signal, and the length of said rod lens is about n/4pitch (where n=1, 3, 5, . . . ).
 19. An optical module for two-waytansmission according to claim 18, wherein said rod lens is insertedinto a tube which has a mark for facilitating packaging, said receivingbase being provided with a guide corresponding to said mark.
 20. Atwo-way optical transmission system in which a plurality of lightsignals of wavelengths differing one from another are transmittedthrough a single optical fiber from one end thereof to the other end andfrom said other end to said one end, comprising:n (where n represents anatural number) first combined units each comprising optical transmittermeans for outputting a light signal of wavelength λ_(i) (i=1, 2, . . .or n), a dielectric thin-film filter for transmitting therethrough thelight signal of the wavelength λ_(i) and reflecting the light signals ofthe other wavelengths and a rod lens for introducing the light signaltransmitted through or reflected by said dielectric thin-film filterinto an optical fiber; m (where m is a natural number) second combinedunits each comprising optical transmitter means for outputting a lightsignal of wavelength λ_(j) (j=1, 2, . . . or m), a dielectric thin-filmfilter for transmitting therethrough the light signal of the wavelengthλ_(j) and reflecting light signals of the other wavelengths and a rodlens for introducing the light signal transmitted through or reflectedby said dielectric thin-film filter into an optical fiber; n thirdcombined units each comprising a rod lens for fetching the light signalof the wavelength λ_(i) from the optical fiber, a dielectric thin-filmfilter for transmitting therethrough the light signal of the wavelengthλ_(i) and reflecting the light signals of the other wavelengths andoptical receiver means for receiving the light signal of the wavelengthλ_(i) transmitted through said dielectric thin-film filter; m fourthcombined units each comprising a rod lens for fetching the light signalof the wavelength λ_(j) from the optical fiber, a dielectric thin-filmfilter for transmitting therethrough the light signal of the wavelengthλ_(j) and reflecting the light signals of the other wavelengths, and anoptical receiver means for receiving the light signal of the wavelengthλ_(j) transmitted through said dielectric thin-film filter; wherein saidn first combined units and said m fourth combined units are disposed onthe side of the one end of said single optical fiber,one end of a givenone of said rod lenses of said first and fourth combined units facingone end of said single optical fiber, and the rod lenses of said firstand said fourth combined units are connected in cascade through opticalfibers; said m second combined units and said n third combined unitsbeing disposed on the side of the other end of said single opticalfiber, one end of a given one of said second and third combined unitsfacing the one end of said single optical fiber, and the rod lenses ofsaid second and third combined units being connected in cascade throughoptical fibers; wherein each of said transmitter means has a lightemission element and a spherical lens, each of said dielectric thin-filmfilters is provided with an angle at an end face of the correspondingrod lens which is located on the entrance side of light emitted by thelight emission element, the length of each of said rod lenses is aboutn/4 pitch (where n=1, 3, 5, . . . ), and each of said transmitter meansand the corresponding filter and rod lens are integrally assembled in asingle receptacle to thereby constitute an optical module.